A communication scheme serving as a successor of W-CDMA and HSDPA, that is, a LTE (Long Term Evolution) system has been and is being discussed by a W-CDMA standardization organization 3GPP. In the LTE system, an OFDM scheme and a SC-FDMA (Single-Carrier Frequency Division Multiple Access) scheme are being investigated as downlink and uplink radio access schemes, respectively. See 3GPP TR 25.814 (V7.0.0), “Physical Layer Aspects for Evolved UTRA”, June 2006, for example.
In the OFDM scheme, a frequency band is segmented into multiple smaller frequency bands (subcarriers), and data is carried and transmitted over the individual subcarriers. According to the OFDM scheme, the subcarriers are densely arranged on the frequency band in such a manner that the subcarriers are partially overlapped with each other without mutual interference, resulting in fast transmission and highly efficient utilization of the frequency band.
In the SC-FDMA scheme, a frequency band is segmented, and different frequency bands are used among multiple terminals for transmission, resulting in reduced interference among the terminals. According to the SC-FDMA scheme, variations in transmit power are reduced, resulting in lower power consumption for the terminals and wider coverage.
The above-mentioned LTE system uses shared channels in uplinks and downlinks. For example, in uplink, a base station apparatus selects mobile stations to communicate to the base station apparatus using the shared channel for each subframe (each 1 ms) and uses downlink control channels to instruct the selected mobile stations to communicate over the shared channel in certain subframes. The mobile stations transmit the shared channels in accordance with the downlink control channels. The base station apparatus receives and decodes the shared channels transmitted from the mobile stations. The above-mentioned selection of mobile stations to communicate over the shared channels is called as scheduling.
Also, AMC (Adaptive Modulation and Coding) is applied to the LTE system, and thus different transmission formats for the shared channels are used for different subframes. The transmission formats may include assignment information of resource blocks, which corresponds to frequency resources, modulation schemes, payload sizes, HARQ related information such as redundancy version parameters and process numbers, and/or MIMO related information such as reference signal sequences for MIMO application, for example.
In the LTE, identification information of the mobile stations communicating over the shared channels in the subframes and the transmission formats of the shared channels are transmitted over physical downlink control channels (PDCCHs). The PDCCH may be also referred to as DL L1/L2 control channels.
In a MAC layer of the LTE, HARQ (Hybrid Automatic Repeat reQeust) is applied in both uplinks and downlinks. For example, the mobile stations transmit acknowledgement information in uplinks based on CRC check results of DL-SCH. The base station apparatus controls retransmissions depending on the acknowledgement information. The acknowledgement information is represented as either of a positive response (ACK) indicating that transmitted signals have been successfully received or a negative response (NACK) indicating that the transmitted signals have not been successfully received. Also, the base station apparatus transmits acknowledgement information in downlinks based on CRC check results of UL-SCH. The mobile station controls retransmission depending on the acknowledgement information. The acknowledgement information is represented as either of a positive response (ACK) indicating that transmitted signals have been successfully received or a negative response (NACK) indicating that the transmitted signals have not been successfully received.